1. Field of the Invention
This invention relates generally to optical transmission, and more specifically to techniques for controlling optical signals in waveguide grating routers.
2. Background of Related Art
Waveguide grating routers have been employed in the field of optical transmission. One example of a waveguide grating router is disclosed in U.S. Pat. No. 5,136,671 (the '671 patent), issued to C. Dragone, and incorporated by reference herein. The waveguide grating router disclosed in the '671 patent is a planar device with N.sub.1 inputs and N.sub.2 outputs arranged in the form of M grated waveguides (i.e. "arms") of varying lengths L(m), connected between two waveguide couplers. The router includes a set of ports p at a first end of the router and a set of ports q at a second end of the router. The router functions as a filter for each input-output (p-q) combination. If the q ports are terminated with an array of N amplifiers and mirrors, and the p ports are terminated with mirrors and optical amplifiers, a multi-frequency laser (MFL) oscillating at N precisely-spaced frequencies is obtained.
In state-of-the-art waveguide grating router designs, the difference in length (.DELTA.L) between any two adjacent waveguides is constant for all waveguides. Mathematically, this may be expressed as .DELTA.L={L(m)-L(m-1)}=k, where k is a constant. In other words, the length L(m) of a particular waveguide (arm) m, is equal to a constant k plus the length of an adjacent arm m-1, denoted as (L(m-1)). The length of the shortest arm, represented by m=1, is a design parameter that is selected in accordance with the desired physical dimensions of the waveguide grating router.
The frequency spectrum of existing waveguide grating routers presents some shortcomings. Within the frequency passband of each channel, a plurality of evenly-spaced frequency components, each at roughly the same peak power level, will occur. The spacing of these frequency components is determined by the router's free-spectral range. For the MFL, in some of the channels, the net power gain may be nearly the same for two or more of these frequency components, especially if the optical amplifiers connected to the MFL router all have substantially similar characteristics. This repetition of the same signal at several different frequencies within the passband of each channel results in multimode lasing of an optical laser, producing instabilities in the laser's output.
A waveguide grating router having a dominant passband (i.e. one predominant frequency component) is therefore highly desirable. In addition, it is desirable to control the particular frequency or passband at which the peak signal occurs within each channel. Note that, as used herein, the term "frequency" may denote a signal having a single frequency, or a signal occupying a given range of frequencies, i.e., a signal passband.